I suppose it depends on what a person considers a hidden gem because everyone's tastes are different. I'll list a few categories and at least 1-2 of my favorites in those categories that I've found on Steam and are currently on sale:

Exploration/Base-Building:

• Grounded
• Raft
• Rimworld
• Subnautica/Subnautica: Below Zero

Story based/Farming/Romance:

• My Time at Portia*
• My Time at Sandrock

Visual Novels:

• Basically anything made by PixelFade (Ace Academy, Ethereal Enigma, Crystalline). One of the few English-based VNs that are fully voiced in English and have a pretty decent plot/story. Interactive with different choices affecting different outcomes and even some minigames at least in EE.
• Nekop er Hush Hush. Ok, listen. I won't beat around the bush here. There IS an 18+ DLC for this but it's not necessary to enjoy the game. It honestly has a pretty interesting story/premise and I did get pretty heavily emotionally invested in the characters.

*Note that MTAP has been fraught with issues. Although these issues don't necessarily make the game unplayable, and it's still enjoyable, be aware that it is was Pathea's (the studio) first such attempt and as such they were still learning. Their second game Sandrock has solved a lot of the issues and it's not necessary to play the first one